Fossil record '', a fossil sponge from the
Cretaceous of England '', a fossil
anthaspidellid demosponge from the early
Ordovician of
Nevada Although
molecular clocks and
biomarkers suggest sponges existed well before the
Cambrian explosion of life,
silica spicules like those of demosponges are absent from the fossil record until the Cambrian. An unsubstantiated 2002 report exists of spicules in rocks dated around . Well-preserved
fossil sponges from about in the
Ediacaran period have been found in the
Doushantuo Formation. These fossils, which include: spicules;
pinacocytes;
porocytes;
archeocytes;
sclerocytes; and the internal cavity, have been classified as demosponges. The Ediacaran record of sponges also contains two other genera: the stem-hexactinellid
Helicolocellus from the
Dengying Formation and the possible stem-archaeocyathan
Arimasia from the
Nama Group. These genera are both from the "Nama assemblage" of Ediacaran biota, although whether this is due to a genuine lack beforehand or preservational bias is uncertain. Fossils of
glass sponges have been found from around in rocks in Australia, China, and Mongolia. Early Cambrian sponges from Mexico belonging to the genus
Kiwetinokia show evidence of fusion of several smaller spicules to form a single large spicule.
Calcium carbonate spicules of
calcareous sponges have been found in Early Cambrian rocks from about in Australia. Other probable demosponges have been found in the Early
Cambrian Chengjiang fauna, from . Fossils found in the Canadian Northwest Territories dating to may be sponges; if this finding is confirmed, it suggests the first animals appeared before the
Neoproterozoic oxygenation event. Freshwater sponges appear to be much younger, as the earliest known fossils date from the Mid-
Eocene period about . The earliest sponge symbionts are known from the
early Silurian. A chemical tracer is
24-isopropyl cholestane, which is a stable derivative of 24-isopropyl
cholesterol, which is said to be produced by
demosponges but not by
eumetazoans ("true animals", i.e.
cnidarians and
bilaterians). Since
choanoflagellates are thought to be animals' closest single-celled relatives, a team of scientists examined the
biochemistry and
genes of one
choanoflagellate species. They concluded that this species could not produce 24-isopropyl cholesterol but that investigation of a wider range of choanoflagellates would be necessary in order to prove that the fossil 24-isopropyl cholestane could only have been produced by demosponges. Although a previous publication reported traces of the chemical 24-isopropyl cholestane in ancient rocks dating to , recent research using a much more accurately dated rock series has revealed that these biomarkers only appear before the end of the
Marinoan glaciation approximately , and that "Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ~ in Oman)". While it has been argued that this 'sponge biomarker' could have originated from marine algae, recent research suggests that the algae's ability to produce this biomarker evolved only in the
Carboniferous; as such, the biomarker remains strongly supportive of the presence of demosponges in the Cryogenian.
Archaeocyathids, which some classify as a type of coralline sponge, are very common fossils in rocks from the Early
Cambrian about , but apparently died out by the end of the Cambrian . It is difficult to fit
chancelloriids into classifications of sponges or more complex animals. An analysis in 1996 concluded that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites ("armor plates") is similar to that of fibers of spongin, a
collagen protein, in modern keratose (horny)
demosponges such as
Darwinella. However, another analysis in 2002 concluded that chancelloriids are not sponges and may be intermediate between sponges and more complex animals, among other reasons because their skins were thicker and more tightly connected than those of sponges. In 2008, a detailed analysis of chancelloriids' sclerites concluded that they were very similar to those of
halkieriids, mobile
bilaterian animals that looked like
slugs in
chain mail and whose fossils are found in rocks from the very Early Cambrian to the Mid Cambrian. If this is correct, it would create a dilemma, as it is extremely unlikely that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related.
Relationships to other animal group In the 1990s, sponges were widely regarded as a
monophyletic group, all of them having descended from a
common ancestor that was itself a sponge, and as the "sister-group" to all other
metazoans (multi-celled animals), which themselves form a monophyletic group. On the other hand, some 1990s analyses also revived the idea that animals' nearest evolutionary relatives are
choanoflagellates, single-celled organisms very similar to sponges'
choanocytes – which would imply that most Metazoa evolved from very sponge-like ancestors and therefore that sponges may not be monophyletic, as the same sponge-like ancestors may have given rise both to modern sponges and to non-sponge members of Metazoa. Analyses since 2001 have concluded that
Eumetazoa (more complex than sponges) are more closely related to particular groups of sponges than to other sponge groups. Such conclusions imply that sponges are not monophyletic, because the
last common ancestor of all sponges would also be a direct ancestor of the Eumetazoa, which are not sponges. A study in 2001 based on comparisons of
ribosome DNA concluded that the most fundamental division within sponges was between
glass sponges and the rest, and that Eumetazoa are more closely related to
calcareous sponges (those with calcium carbonate spicules) than to other types of sponge. Other anatomical and biochemical evidence links the Eumetazoa with
Homoscleromorpha, a sub-group of demosponges. A comparison in 2007 of
nuclear DNA, excluding glass sponges and
comb jellies, concluded that: •
Homoscleromorpha are most closely related to Eumetazoa; • calcareous sponges are the next closest; • the other demosponges are evolutionary "aunts" of these groups; and • the
chancelloriids, bag-like animals whose fossils are found in
Cambrian rocks, may be sponges. The
sperm of Homoscleromorpha share features with the sperm of Eumetazoa, that sperm of other sponges lack. In both Homoscleromorpha and Eumetazoa layers of cells are bound together by attachment to a carpet-like basal membrane composed mainly of "typ IV"
collagen, a form of collagen not found in other sponges – although the
spongin fibers that reinforce the mesohyl of all demosponges is similar to "type IV" collagen. The analyses described above concluded that sponges are closest to the ancestors of all Metazoa, of all multi-celled animals including both sponges and more complex groups. However, another comparison in 2008 of 150 genes in each of 21 genera, ranging from fungi to humans but including only two species of sponge, suggested that
comb jellies (
ctenophora) are the sister group to all other lineages of Metazoa included in the sample. If this is correct, either modern comb jellies developed their complex structures independently of other Metazoa, or sponges' ancestors were more complex and all known sponges are drastically simplified forms. The study recommended further analyses using a wider range of sponges and other simple Metazoa such as
Placozoa. 'Family trees' constructed using a combination of all available data – morphological, developmental and molecular – concluded that the sponges are in fact a monophyletic group, and with the
cnidarians form the sister group to the bilaterians. A very large and internally consistent alignment of 1,719 proteins at the metazoan scale, published in 2017, showed that (i) sponges – represented by Homoscleromorpha, Calcarea, Hexactinellida, and Demospongiae – are monophyletic, (ii) sponges are sister-group to all other multicellular animals, (iii) ctenophores emerge as the second-earliest branching animal lineage, and (iv)
placozoans emerge as the third animal lineage, followed by
cnidarians sister-group to bilaterians. In March 2021, scientists from Dublin found additional evidence that sponges are the sister group to all other animals, while in May 2023, Schultz et al. found patterns of irreversible change in genome synteny that provide strong evidence that
ctenophores are the sister group to all other animals instead. == Notable spongiologists ==